Fiber-optic cables carry hundreds of glass strands and function as a highway for internet traffic. File/AP

A few Charleston internet users were still struggling to connect early Wednesday as the fallout from a severed fiber optic cable began to fade.

The online disconnect lasted about 12 hours, cutting off thousands of business and residential customers, who are not likely to learn much more about the cause and whether another can be prevented.

It started at about 10:30 a.m., affecting Internet access offered through WOW!, Comcast and Verizon. The majority of the issues were fixed slightly before midnight. Landline phones were reportedly affected as well, as were customers in the Raleigh area. 

Tuesday's problems originated with a cut to one of CenturyLink's fiber optic lines, which are made up of hundreds of strands of glass that transport internet traffic. 

The Louisiana-based company with about 450,000 miles of fiber optic broadband networks confirmed in a tweet late Tuesday the outage was due to a cut on their network. At 11:30 p.m., the company said on social media that technicians were still working to address the problem.

As of mid-morning Wednesday, most of the service in Charleston was restored, said Frank Tutalo, a spokesman for CenturyLink. A third party caused the damage, he said in an email, but he didn't elaborate about how it happened.

He also declined to answer questions about precisely where the line was cut, what redundancies are in place to prevent outages and how many customers were affected.

Tutalo did say that fiber cuts are more common this time of year because the uptick in construction activity and roadwork.

Telecommunications companies are required to report outages to the Federal Communications Commission, but as a policy the FCC does not release that information to the public.

Kuang-Ching Wang, a Clemson engineer who researches wireless networks, said, ideally, cable network take multiple paths so that if one is somehow disabled, another is available as a backup.

At Clemson's main Upstate campus, for instance, fiber networks run east and west — one toward Atlanta, the other toward Charlotte — creating a redundant system.

But it can be tough for companies to predict and plan for every possible scenario, he said.

“Sometimes, based on engineering limitations, there can be some bottlenecks where it’s not easy for you to have redundancy,” Wang said.

Each internet service provider also has different configurations.

WOW! spokeswoman Debra Havins said engineers explored ways to reroute internet traffic while they made repairs Tuesday night.

“Unfortunately, in this case, the damage was extensive and even impacted redundant routes,” she said.

WOW! connects more than 90,000 homes and businesses in the Charleston area. The company sent a note to its customers saying "extensive repairs were required through the night and early morning." It also offered customers a $5 credit for the trouble.

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Comcast said if one its own fiber lines is severed, backup systems would keep its services operating. Spokesman Alex Horwitz said the Philadelphia-based cable TV and internet giant operates its own network in certain areas, and in those places, "we have redundant technology that generally assures our customers will continue to enjoy service, even in the event of a fiber cut."

Horwitz said in an email that he didn't know precisely what caused Tuesday's damage, only that a network serving both North Carolina and South Carolina was cut.

"In these types of events, Comcast must wait on the impacted provider to make the necessary repairs," he said.

Networks can be damaged if machine operators aren't aware of or don't see cable either underground or overhead. The many miles of fiber are not always clearly marked, Clemson's Wang said. 

In the event of a cut, technicians have to use complicated maps to pinpoint the origin. Wang said fiber cables are connected to routers as large as a refrigerator that can be miles away from each other. Technicians first must find the right box, and then track down where the line is severed.

The services of least one other well-known internet company reported similar technical issues as WOW!, Comcast and Verizon. Search giant Google said in a status report Tuesday night that its services could be dragging for some customers because of physical damage to multiple fiber bundles.

The company, which runs a major data center in Berkeley County, did not say whether the problem was related to the cut to the CenturyLink network.

Tuesday's disruption was widespread but it was not universal.

The Town of Kiawah said in a notice to residents that it had to use a backup internet system through AT&T.

But Charleston County, which operates the area's 911 call center, uses AT&T and Spirit and had no problems, a spokesman said. Charleston International Airport also was not affected, CEO Paul Campbell said.

Reach Mary Katherine Wildeman at 843-937-5594. Follow her on Twitter @mkwildeman.